Re: perf with precise attribute kills all KVM based VMs

[David Ahern <dsahern@gmail.com>]

On 7/11/12 3:53 AM, Gleb Natapov wrote:
> On Wed, Jul 11, 2012 at 11:49:47AM +0200, Peter Zijlstra wrote:
>> On Wed, 2012-07-11 at 10:10 +0300, Gleb Natapov wrote:
>>
>>> Looks like Avi is right about the overshoot. Can you test something like this?
>>>
>>> diff --git a/arch/x86/kernel/cpu/perf_event_intel.c b/arch/x86/kernel/cpu/perf_event_intel.c
>>> index 166546e..5fb371a 100644
>>> --- a/arch/x86/kernel/cpu/perf_event_intel.c
>>> +++ b/arch/x86/kernel/cpu/perf_event_intel.c
>>> @@ -1374,8 +1374,11 @@ static struct perf_guest_switch_msr *intel_guest_get_msrs(int *nr)
>>>   	arr[0].msr = MSR_CORE_PERF_GLOBAL_CTRL;
>>>   	arr[0].host = x86_pmu.intel_ctrl & ~cpuc->intel_ctrl_guest_mask;
>>>   	arr[0].guest = x86_pmu.intel_ctrl & ~cpuc->intel_ctrl_host_mask;
>>> +	arr[1].msr = MSR_IA32_PEBS_ENABLE;
>>> +	arr[1].host = cpuc->pebs_enabled;
>>> +	arr[1].guest = 0;
>>> +	*nr = 2;
>>>
>>> -	*nr = 1;
>>>   	return arr;
>>>   }
>>

So far the 64-bit Fedora 10 VM with both a Fedora 10 stock kernel and a 
2.6.38 kernel have not faired well - and that's the only VM I have tried 
at the moment. Using -e cycles:pp I have been able to lock up the VM 3 
times out of 3 series of tests with perf-kvm that includes network 
traffic (e.g., netperf), disk I/O (dd based to create a file with dsync 
flag) and pure userspace cpu bound (openssl speed). May or may not be 
related.

Also, I noted that 'perf kvm --guest record -e cycles:pp' does not 
generate a whole lot of samples -- like < 100 in a 20-second sample -- 
despite the fact that the guest is rather busy.

I won't have much time over the next few days to run much in the way of 
tests; I'll come back to it Sunday night.

David

>>
>> You also need to clear TR, BTS, BTINT from MSR_IA32_DEBUGCTLMSR and
>> ideally you'd also clear MSR_IA32_DS_AREA so that any write will be a
>> proper NULL deref or such.
> Yes. With the patch above :pp modifier does not crash guest for me, but
> in theory it should since BTS are still written to DS. May be BTS writes do
> not overshoot guest entry. Will have to ask Intel for clarification.